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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

396

ARACHIS OIL

shaken until a clear solution is obtained, this being allowed to cool. When
the temperature has fallen somewhat, slender silvery needles begin to form
and rapidly collect into tufts (lignoceric acid), together with gradually
increasing shining, nacreous leaflets (arachidic acid). With the fatty acids
from pure arachis oil, the temperature at which crystals begin to form as
the alcoholic solution cools is 35-38. The melting point of the mixture
of acids obtained in this first crystallisation is usually 71-73.

No other oil gives a crystalline precipitate in such conditions, even
when the alcoholic solution of its solid acids (prepared as above) is cooled
to the ordinary temperature. Only cottonseed oil and some olive oil from
Tunisian olives give a precipitate, but this is amorphous and granular and
in perfectly opaque mammillary masses, m.p. below 70 ; further, such
precipitate does not form in a second crystallisation.

If it is required to estimate exactly the quantity of arachidic and lig-
noceric acids, the crystals formed in the alcoholic solution are collected
and washed with three successive quantities of 10 c.c. of 90% alcohol and
then thoroughly with 70% alcohol. The crystals are then redissolved in
100 c.c. of 90% alcohol (or a less volume if the amount is small) and the
crystallisation repeated as described above. The crystals thus obtained
are collected on a filter, washed twice with 10 c.c. of 90% alcohol and then
with 70% alcohol until this dissolves no more ; they are then dissolved in
a little boiling absolute alcohol, the solution being evaporated in a tared
dish and the residue dried at 100 for about an hour and weighed.1 To
the weight found is added that of the arachidic and lignoceric acids remaining
in solution in the 90% alcohol used for the various crystallisations and
washings, the following solubility coefficients (Tortelli and Ruggeri) being
employed:


	Number of grams dissolved by 100 c.c. of 90% alcohol at a
		
Weight of Acid
	temperature of
		

	15
	17-5
	20

I -oo or more
	0-071
	0-081
	0-09I

0-70 .....
	0-068
	0-078
	0-089

0-50 .....
	0-064
	0-075
	0-084

0-25 .....
	0-052
	0060
	0-067

0-05 or le;,s
	0-031
	0-040
	0-045

Arachis oil contains, on the average, 4-80% of arachidic and lignoceric acids
together, so that the proportion of these acids found by the above method
indicates if the oil is pure or not. Further, the presence and quantity of these
acids serve for the characterisation and determination of arachis oil in mixtures
with olive and other oils. In such mixtures, the crystallisation of the arachidic
and lignoceric acids from the alcoholic solution of the solid fatty acids takes
place at lower and lower temperatures as the proportion of arachis oil in the
mixture diminishes.

1 The mixture of arachidic and lignoceric acids thus obtained should melt at 74-

75 "5" olive oil a solid white or yellowish mass is obtained and similar